As to methods for producing low-silica faujasite type zeolite (hereinafter also referred to as "LSX") having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 1.9 to 2.1, various methods have been disclosed. For example, GB 1580928 discloses a method of crystallizing a mixture containing sodium, potassium, an aluminate and a silicate at a temperature of 50.degree. C. or less, or aging the mixture at a temperature of 50.degree. C. or less for an extremely long time of 15 to 72 hours, followed by crystallization within the temperature range of 60 to 100.degree. C.
ZEOLITES, vol. 7, September, pages 451-457 (1987) discloses the influence of the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio, K/(Na+K) molar ratio, aging conditions and crystallization conditions of raw materials on the formation of low-silica faujasite type zeolite in detail, and the aging and crystallization are performed in a plastic vessel.
Further, U.S. Pat. No. 4,859,217 discloses a method of mixing a mixture containing sodium, potassium and an aluminate with a mixture containing a silicate at a low temperature of 4 to 12.degree. C. to allow the resulting mixture to gel, aging the gel at 36.degree. C. for 48 hours, and then elevating the temperature thereof to 70.degree. C. to conduct crystallization, and it is clearly shown that the cooling in the final mixing and the avoidance of the occurrence of excessive mechanical energy are important.
As described above, in the production of low-silica faujasite type zeolite, it has been considered indispensable that the solution containing sodium, potassium, an aluminate and a silicate is mixed it low temperature to allow the solution to gel, the gel is aged in the still standing state for a long period of time, and the temperature thereof is elevated to the crystallization temperature in the still standing state to conduct crystallization. However, it is industrially disadvantageous to cool the raw material to low temperature, and the gel is very poor in heat transfer characteristics. In large-scale synthesis, therefore, there is the difficulty that an extremely large period of time is required for homogenizing the temperature in the still standing state.